This invention relates to a method and apparatus for real time optical image processing, i.e., for computing and processing images utilizing a passive phase conjugate mirror.
There has been a growing interest in real-time optical image processing for convolving and correlating objects with spatial information, performing inversion of gray-scale objects, subtraction, and differentiation. In recent years, photorefractive crystals have been used to perform such real-time image processing operations. For example, real-time convolution and correlation have been demonstrated by J. O. White and A. Yariv, "Real-time image processing via four-wave mixing in a photorefractive medium," Appl. Phys. Lett., Vol. 37, pp 5-7 (1980). Edge enhancement has been reported by J. Feinberg, "Realtime edge enhancement using photorefractive effect," Opt. Lett., Vol. 5, pp 330-332 (1980). Work related to image subtraction has also been reported by J. P. Huignard, J. P. Herrian, and F. Micherson, "Coherent selective erasure of superimposed volume holograms in LiNbO.sub.3," Appl. Phys. Lett., Vol. 26, pp 225-258 (1975) and by Y. H. Ja, "Real-time image subtraction in four-wave mixing with photorefractive Bi.sub.12 GeO.sub.20 crystal," Opt. Comm., Vol. 42, pp 377-388 (1982). The latter employed two sequential exposures of the hologram and was thus not strictly real-time. Differentiation, division and inversion have also been reported by Y. H. Ja., "Real-time optical image differentiation by degenerate four-wave mixing," Appl. Phys. B., Vol. 36 pp 21-24 (1985), "Real-time image division in four-wave mixing with photorefractive Bi.sub.12 GeO.sub.20 crystal, " Opt. Comm., Vol. 44, pp 24-28 (1982), and E. Ochoa, et al., "Real-time intensity inversion using two waves and four-wave mixing in photorefractive Bi.sub.12 GeO.sub.20," Appl. Opt., Vol. 24, pp 1826-32 (1985), respectively. An object of this invention is to provide simple, exact and high performance optical systems for real-time image processing utilizing a passive (self-pumped) phase conjugate mirror which reflects back through the medium a coherent beam on the axis of the incident beam, as disclosed in U.S. Pat. No. 4,529,273.